The present invention is directed to balancing centrifuge rotors and, more particularly, is directed to an improved rotor structure to permit balancing without affecting the strength of the rotor.
Critical to the successful operation of a high speed ultracentrifuge is the precise balance in the rotor for its rotation about its spin axis. When a rotor operates at a speed of several thousand r.p.m.'s, any inherent imbalance in the rotor will result in unstable operation. The consequences of unstable operation can be quite serious. The rotor may become detached from its drive spindle, causing damage not only to the rotor, but also to the centrifuge machine in which it is placed. Furthermore, valuable samples carried in the rotor may be lost.
One method of making centrifuge rotors is to forge the rotor out of a high strength metal and then machine all or part of the rotor to establish the proper balance. Because machining is very expensive, any reduction in amount of machining necessary on a rotor is greatly desirable. Unfortunately, in most forged surfaces, there are significant tolerance variations which would create unacceptable imbalance in a rotor. Typically the outer surface of the rotor is machined to provide the desired balance in the rotor. However, in high speed ultracentrifugation the highest stress in the rotor is at the maximum diameter of the rotor. In other words, at the extreme periphery where the distance from the spin axis is the greatest will be the location of the most stress in the rotor. It is, therefore, undesirable to remove material from the exterior of the rotor, since it may affect the integral strength of the rotor during high speed ultracentrifugation. This is especially true with respect to rotors designed for a minimum of machining.
In the initial design of the rotor the dimensions of the rotor body with respect to the cavities or recesses for receipt of the containers having the fluid samples are precisely defined to a close tolerance. This tolerance is important with respect to not using any more material than is necessary to provide the requisite strength to support the fluid samples. Otherwise, if too great a mass is utilized in the design of the rotor, its maximum safe speed is reduced for the same size drive system. Also, the size of the rotor is generally limited by its interface with the rotor chamber in the centrifuge.
In certain particular rotors, which are sometimes referred to as swinging bucket rotors, a plurality of arms extend radially outward from a central post to which is connected the drive spindle. The outer extremeties of the arms are connected to a support ring. Swinging bucket containers are positioned between the arms and, during centrifugation, the buckets will swing from a vertical position to a horizontal position. Furthermore, the buckets are designed to seat on the interior surface of this outer ring. Consequently, this outer ring provides essentially all the support to the swinging buckets during centrifugation. Thus, it is undesirable to machine any of the surface of this outer ring, because it may affect its integral strength during centrifugation.
As stated previously, in the initial design of any rotor the mass and dimensions of the rotor are carefully formulated to obtain the optimum support while limiting the amount of stress experienced by the rotor for a given speed. In any event, there are particular areas in the rotor that are exposed to extremely high stress. These high stress areas are typically at the outer extremity of the rotor. The state of the art of forging is limited in the capability of making a perfectly balanced rotor. Therefore, some machining will always be required to obtain the precise balance needed for high speed centrifugation. The achievement of the perfect balance, however, cannot be accomplished at the sacrifice of the requisite strength in the rotor to withstand the high stresses during centrifugation.